systemd.journal-fields man page

Entries in the journal resemble an environment block in their syntax but with fields that can include binary data. Primarily, fields are formatted UTF-8 text strings, and binary formatting is used only where formatting as UTF-8 text strings makes little sense. New fields may freely be defined by applications, but a few fields have special meaning. All fields with special meanings are optional. In some cases, fields may appear more than once per entry.

The human-readable message string for this entry. This is supposed to be the primary text shown to the user. It is usually not translated (but might be in some cases), and is not supposed to be parsed for metadata.

A 128-bit message identifier ID for recognizing certain message types, if this is desirable. This should contain a 128-bit ID formatted as a lower-case hexadecimal string, without any separating dashes or suchlike. This is recommended to be a UUID-compatible ID, but this is not enforced, and formatted differently. Developers can generate a new ID for this purpose with journalctl --new-id128.

Syslog compatibility fields containing the facility (formatted as decimal string), the identifier string (i.e. "tag"), and the client PID. (Note that the tag is usually derived from glibc's program_invocation_short_name variable, see program_invocation_short_name(3).)

The process, user, and group ID of the process the journal entry originates from formatted as a decimal string. Note that entries obtained via "stdout" or "stderr" of forked processes will contain credentials valid for a parent process (that initiated the connection to systemd-journald).

The control group path in the systemd hierarchy, the the systemd slice unit name, the systemd unit name, the unit name in the systemd user manager (if any), the systemd session ID (if any), and the owner UID of the systemd user unit or systemd session (if any) of the process the journal entry originates from.

The earliest trusted timestamp of the message, if any is known that is different from the reception time of the journal. This is the time in microseconds since the epoch UTC, formatted as a decimal string.

Only applies to "_TRANSPORT=stdout" records: specifies a randomized 128bit ID assigned to the stream connection when it was first created. This ID is useful to reconstruct individual log streams from the log records: all log records carrying the same stream ID originate from the same stream.

Only applies to "_TRANSPORT=stdout" records: indicates that the log message in the standard output/error stream was not terminated with a normal newline character ("\n", i.e. ASCII 10). Specifically, when set this field is one of nul (in case the line was terminated by a NUL byte), line-max (in case the maximum log line length was reached, as configured with LineMax= in journald.conf(5)) or eof (if this was the last log record of a stream and the stream ended without a final newline character). Note that this record is not generated when a normal newline character was used for marking the log line end.

The kernel device name. If the entry is associated to a block device, the major and minor of the device node, separated by ":" and prefixed by "b". Similar for character devices but prefixed by "c". For network devices, this is the interface index prefixed by "n". For all other devices, this is the subsystem name prefixed by "+", followed by ":", followed by the kernel device name.

These are additional fields added automatically by systemd-journald. Their meaning is the same as _UID=, _GID=, _COMM=, _EXE=, _CMDLINE=, _AUDIT_SESSION=, _AUDIT_LOGINUID=, _SYSTEMD_CGROUP=, _SYSTEMD_SESSION=, _SYSTEMD_UNIT=, _SYSTEMD_USER_UNIT=, and _SYSTEMD_OWNER_UID= as described above, except that the process identified by PID is described, instead of the process which logged the message.

During serialization into external formats, such as the Journal Export Format[1] or the Journal JSON Format[2], the addresses of journal entries are serialized into fields prefixed with double underscores. Note that these are not proper fields when stored in the journal but for addressing metadata of entries. They cannot be written as part of structured log entries via calls such as sd_journal_send(3). They may also not be used as matches for sd_journal_add_match(3)

The wallclock time (CLOCK_REALTIME) at the point in time the entry was received by the journal, in microseconds since the epoch UTC, formatted as a decimal string. This has different properties from "_SOURCE_REALTIME_TIMESTAMP=", as it is usually a bit later but more likely to be monotonic.

The monotonic time (CLOCK_MONOTONIC) at the point in time the entry was received by the journal in microseconds, formatted as a decimal string. To be useful as an address for the entry, this should be combined with the boot ID in "_BOOT_ID=".